Low-power voltage-controlled oscillator

ABSTRACT

In one embodiment, a voltage-controlled oscillator (VCO) is provided that includes: a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source, the current source sourcing the tail current responsive to a bias voltage, wherein each transistor in the differential pair couples to a power source through a corresponding switching-capacitor circuit; and a bias circuit configured to generate the bias voltage such that a transconductance for each transistor in the differential pairs is proportional to a factor that is a function of a ratio of transistor widths within the bias circuit.

TECHNICAL FIELD

The present invention relates generally to voltage controlled oscillators and more particularly to a voltage controlled oscillator whose start up conditions are independent of process corners, temperatures, and supply voltage.

BACKGROUND

Ring oscillators are popular in applications such as voltage controlled oscillators (VCOs), clock generation in microprocessor, carrier synthesis in cellular phones, phase-locked loops, and data and clock recovery circuits. A ring oscillator is denoted as a free-running oscillator in that no timing signal or control signal is used to initiate the oscillation. Ring-oscillator-implemented VCOs and related free-running oscillator components are among the most power-hungry components in any electronic system. Thus low-power VCO design poses fundamental challenges.

Accordingly, there is a need in the art for improved low-power VCO designs.

FIG. 2 illustrates a typical implementation for differential inverter stages 100. A differential pair of NMOS transistors Q1 and Q2 have their drains isolated from a supply voltage node V_(CC) by PMOS transistors M2 and M3, respectively. Each PMOS transistor M2 and M3 has its gate controlled by the control voltage signal V_(control) such that transistors M2 and M3 act as resistors in the triode mode of operation.

Thus, the amplitude of the control voltage controls the resistance through transistors M2 and M3 and hence the signal delay in each inverter stage. Each transistor M2 and M3 may thus be represented by a variable resistor of resistance R determined by the control voltage. Differential input voltages V_(in) ⁺ and V_(in) ⁻ control the gates of transistors Q1 and Q2, whose sources are tied to a current source driving the tail current I. The drains of transistors Q2 and Q1 tie to the nodes for differential output voltages V_(out) ⁺ and V_(out) ⁻, respectively. Because transistors Q1 and Q2 form a differential pair, virtually the entire tail current I will steer through the transistor whose gate voltage is higher than a threshold voltage multiple as compared to the remaining gate voltage. For example, if V_(in) ⁺ is sufficiently higher than V_(in) ⁻, the tail current steers through Q1. The output voltage V_(out) ⁺ will thus be at V_(CC) whereas V_(out) ⁻ will be at V_(CC)−I*R, where R is the resistance of M2 and I is the tail current. These output voltages switch if V_(in) ⁻ is sufficiently higher than V_(out) ⁺. The amplitude of the output signal is thus I*R. It can be shown that the output frequency of voltage-controlled oscillator 100 is proportional to the inverse of the propagation delay τ for each inverter stage 101. In turn, the delay is proportional to resistance R through transistors M2 and M3. Thus, the output frequency is a nonlinearly dependent on the control voltage because the control voltage controls the resistance R. It follows that the output frequency is nonlinearly dependent on the output amplitude.

This nonlinear dependence is undesirable because as the voltage-controlled oscillator is tuned across its frequency range, the output signal's amplitude will vary significantly. Accordingly, there is a need in the art for voltage-controlled oscillators having output signals with independent frequency and amplitude.

SUMMARY

In one embodiment, a ring oscillator is provided that includes: a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source, the current source sourcing the tail current responsive to a bias voltage; and a bias circuit configured to generate the bias voltage such that a trans conductance for each transistor in the differential pairs is proportional to a factor that is a function of a ratio of device widths within the bias circuit.

In another embodiment, a voltage-controlled oscillator (VCO is provided that includes: a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source, the current source sourcing the tail current responsive to a bias voltage, wherein each transistor in the differential pair couples to a power source through a corresponding switching-capacitor circuit; and a bias circuit configured to generate the bias voltage such that a transconductance for each transistor in the differential pairs is proportional.

In another embodiment, a phase-locked loop (PLL) is provided that includes: a phase detector configured to compare the phase between a divided signal and in input signal to provided a phase detector output; a loop filter to filter the phase detector output to provide a tuning voltage; and a voltage-controlled oscillator (VCO) including a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source, the current source sourcing the tail current responsive to a bias voltage, wherein each transistor in the differential pair couples to a power source through a corresponding switching-capacitor circuit having an adjustable capacitance responsive to the tuning voltage; and a bias circuit configured to generate the bias voltage such that a transconductance for each transistor in the differential pairs is proportional to a factor that is a function of a ratio of transistor widths within the bias circuit.

The invention will be more fully understood upon consideration of the following detailed description, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a diagram of a 3-stage ring oscillator.

FIG. 1 b is a diagram of a 4-stage ring oscillator.

FIG. 2 is a circuit diagram for a differential inverter stage in the ring oscillators of FIGS. 1 a and 1 b.

FIG. 3 is a circuit diagram for a biasing circuit to bias the current source transistor in a differential inverter stage.

FIG. 4 a is a circuit diagram for a conventional VCO stage.

FIG. 4 b is a circuit diagram for a VCO stage configured to use switched-capacitor circuits to make the VCO frequency of oscillation independent of process corner and temperature variations.

FIG. 5 a illustrates a conventional resistance.

FIG. 5 b illustrates a switched-capacitor circuit providing an equivalent resistance.

FIG. 5 c illustrates a switched-capacitor circuit providing an equivalent resistance that is parasitic insensitive.

FIG. 6 is a circuit diagram for a biasing circuit to bias the current source transistor in a VCO stage.

FIG. 7 is a circuit diagram of the VCO stage from FIG. 4 b modified to use the biasing circuit of FIG. 6.

FIG. 8 shows a VCO including a plurality of stages as disclosed in FIG. 7.

FIG. 9 shows a phase-locked loop (PLL) incorporating the VCO of FIG. 8.

Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

Reference will now be made in detail to one or more embodiments of the invention. While the invention will be described with respect to these embodiments, it should be understood that the invention is not limited to any particular embodiment. On the contrary, the invention includes alternatives, modifications, and equivalents as may come within the spirit and scope of the appended claims. Furthermore, in the following description, numerous specific details are set forth to provide a thorough understanding of the invention. The invention may be practiced without some or all of these specific details. In other instances, well-known structures and principles of operation have not been described in detail to avoid obscuring the invention.

To provide a low-power VCO whose startup conditions are independent of process corner and temperature, a ring-oscillator having the same independence for its startup conditions will first be addressed.

Free-Running Oscillator

A ring oscillator typically includes an odd number of inverters coupled together in a loop. To eliminate common-mode noise, it s conventional to use differential or double-ended inverters as shown in FIG. 1( a). In contrast, if an even number of inverters are coupled together in this fashion, a latch results unless one of the stages is configured to be non-inverting as shown in FIG. 1( b). To form a voltage-controlled oscillator (VCO), each inverter stage in a ring oscillator is configured so that its propagation delay is responsive to a control voltage.

A conventional differential inverter stage 200 is shown in FIG. 2. Stage 200 uses NMOS transistors M₁, M₂ and M₃ but it will be appreciated that stage 200 may be implemented using an equivalent PMOS embodiment. M₃ acts as a current source with regard to a tail current I. Transistors M₁ and M₂ form a differential pair of transistors that steer the tail current I responsive to differential input voltages V₁ and V₂. As V₁ is made higher than V₂ by a multiple of the threshold voltage V_(T) for transistors M₁ and M₂, the tail current I will conduct virtually entirely through M₁. Similarly, as V₂ gets higher than V₁, the tail current I steers over to M₂ to conduct virtually entirely through M₂. During oscillation, the voltage V₁ and V₂ successively switch in this fashion to continually steer the tail current I back and forth between transistors M₁ and M₂.

This oscillation occurs when a product of the transconductance for M1 and M2 (since these transistors are matched, their common transconductance may be denoted as g_(m1,2)) and load resistance R exceeds or equals 1 (the well-known Barkhausen stability condition for oscillation). The transconductance g_(m1,2) thus plays a key role in determining the startup condition for a free-running oscillator (and any VCO containing such an oscillator). In any differential pair of transistors M₁ and M₂, their transconductance is proportional to the transconductance for the current-source transistor M₃ (which may be denoted as g_(m3)). Thus, the transconductance g_(m1,2) equals A·g_(m3) where A is a proportionality constant. In particular, if M₁, M₂ and M₃ all have the same width and length, then g_(m1,2)=g_(m3)/sqrt(2) since at equilibrium, the current through M₁ and M₂ is half the curren through M₃, where sqrt represents the square root function.

To provide a low power VCO that is insensitive to process corners and temperature variations, the transconductance g_(m3) for M₃ is made proportional to 1/R, where R is the load resistance. As will be discussed further herein, the startup condition for oscillation then becomes just a ratio of resistances and hence becomes independent of temperature and process corner.

Turning now to FIG. 3, a supply-independent biasing circuit 300 is shown that achieves this advantageous biasing. Biasing circuit includes a pair of PMOS transistors N₄ and N₅ in a current mirror relationship such that the drain and gate for transistor N₅ as well as the gate for N₄ have the same voltage. Since PMOS transistors N₄ and N₅ form a current mirror, their dimensions as defined through a width (W) and a length (L) ratio W/L are matched. The drain of N₄ couples to a drain for an NMOS transistor M₄ whose source couples to ground. M₄ also has the same W/L ratio. The drain and gate of M₄ are coupled together to force M₄ into saturation responsive to a gate voltage V_(M4). The drain of N₅ couples to a drain of another NMOS transistor M₅ whose source couple to ground through a resistance R. In contrast to the other transistors, M₅ has a W/L ratio that is a factor X times larger. But for this size difference, transistor M₄ and M₅ would also be in a current mirror relationship. It may be shown that the transconductance for M₄ (denoted as g_(m4)) equals (2/R)·(1−l/sqrt(X)). The transconductance for M ₄ is thus equaling the desired proporationality to a resistance as discussed above with regard to current source transistor M₃. Thus, if the dimensions for M₃ are made to match those for M₄ (same W/L ratio) and if M₃ has the same biasing voltage (setting V_(M3) equal to V_(M4) as shown in FIG. 2), then the transconductance g_(m3) also equals (2/R)·(1−l/sqrt(X)). As discussed previously, the transconductance g _(m1,2) equals A·g_(m3).

The startup condition of g_(m1,2)·R≧1 thus becomes 2A(1−l/sqrt(X))≧1. Such a startup condition depends only on the ratio of the widths for transistors N₅ and M₅, which is plainly independent of process corners and temperature. The adaptation of such a free-running oscillator into a VCO will now be addressed.

Voltage Controlled Oscillator (VCO)

Like a ring oscillator, a VCO also includes a plurality of VCO stages coupled together into a loop analogously as discussed with regard to FIGS. 1 a and 1 b. However, unlike the fixed load resistances R shown for inverter 200 of FIG. 2, the stages for a VCO have a variable resistance so that the frequency of oscillation may be voltage controlled. An example VCO stage 400 is shown in FIG. 4 a. Stage 400 includes transistors M₁, M₂, and M₃ as discussed with regard to inverter 200. For generality, however, the gate voltage for M₃ is denoted as V_(cntl). A parasitic capacitance between stages is denoted as C_(L). It may be seen that inverter stage 400 differs from inverter 200 in that load resistances R_(M) are adjustable. The resulting voltage-controlled oscillation frequency f_(VCO) is proportional to 1/(R_(M)·C_(L)). This frequency of oscillation can be made independent of process corner and temperature variations as follows.

The independence stems from the equivalence of resistors and switched-capacitor circuits. This equivalence may be better understood with reference to FIGS. 5 a, 5 b, and 5 c. FIG. 5 a shows a resistor of resistance R_(m) connected between two nodes. For a given amount of voltage potential between nodes A and B, resistance R_(m) functions to allow the transfer of a certain amount of charge every second from node A to node B. The same function can also be performed by a switched-capacitor circuit 500 shown in FIG. 5 b. Switched capacitor circuit 500 includes a capacitor of C_(clk) having one terminal coupled to ground and a remaining terminal alternatively coupled to each of the two nodes through switches S₁ and S₂. Switches S₁ and S₂ are opened and closed in non-overlapping opposition to each other according to a clock rate f_(clk). For example, during a first half cycle of the clock, switch S₁ is closed and switch S₂ is open. During a subsequent half cycle of the clock, switch S₁ is opened and switch S₂ is closed. The charge moved from node A to node B in one clock period is equal to the average current flowing between the two nodes, i.e.,

I = f_(clk)C_(clk)(V_(A) − V_(B)) or $R_{m} = {\frac{1}{f_{clk}C_{clk}}.}$

Therefore, switched-capacitor circuit 500 may be viewed as a resistor whose value is equal to

$\frac{1}{f_{clk}C_{clk}}.$

The equivalence between resistors and switched-capacitors can be made more precise by using extra switches S₃ and S₄ in a switched-capacitor circuit 505 as shown in FIG. 5 c to provide a parasitic-insensitive property. Switches S₂ and S₄ are opened and closed in parallel. Similarly, switches S₁ and S₃ are opened and closed in parallel in non-overlapping opposition to switches S₂ and S₄ according to clock rate f_(clk). The equivalent resistance of a switched-capacitor circuit thus depends on the clocking rate f_(clk) or the capacitance C_(clk). Adjusting either factor adjusts the equivalent resistance. FIG. 5 d illustrates an adjustable switched-capacitor circuit in which the fixed capacitance of FIG. 5 b is replaced by a varactor having an adjustable capacitance in response to control voltage V_(CNTL). It will be appreciated that an equivalent adjustable switched capacitor could be implemented using a fixed capacitance and an adjustable clocking signal. Similarly, both the clocking frequency and the capacitance may be made adjustable.

Given this equivalence, adjustable resistances Rm in VCO stage 400 may be replaced with switched-capacitor circuits 405 for a VCO stage 410 as shown in FIG. 4 b. Replacing resistors R_(m) with their equivalent switched-capacitor circuits leads to the following advantageous VCO frequency of oscillation (f_(VCO)):

$f_{vco} \propto {\frac{f_{clk}C_{clk}}{C_{L}}.}$

VCO stage 410 thus enables a VCO oscillation frequency that depends only on the ratio of capacitors. Whatever process corner dependence and temperature dependence that exists in each capacitance will thus cancel out in such a ratio, providing an intrinsic self-compensation that makes f_(VCO) independent of temperature and process variations.

Not only can the oscillation frequency be made insensitive to temperature and and process corners variations as just discussed but the startup conditions can also be made independent using an analogous biasing circuit as discussed with regard to FIG. 3. As seen in FIG. 6, biasing circuit 600 is modified with regard to biasing circuit 300 of FIG. 3 by replacing the resistance R in circuit 300 with a switched-capacitor circuit 605. It thus follows that the transconductance of M₄ in biasing circuit 600 becomes

$g_{m\; 4} = {2\left( {1 - \frac{1}{\sqrt{X}}} \right)f_{clk}{C_{clk}.}}$

An advantageous property for this transconductance is the factor

$2\left( {1 - \frac{1}{\sqrt{X}}} \right)$

in front of the factor f_(clk)·C_(clk). The clock frequency can therefore be a factor of

$2\left( {1 - \frac{1}{\sqrt{X}}} \right)$

(typically as much as 40%) less than other methods.

FIG. 7 shows a VCO inverter stage 700 that includes biasing circuit 600. By setting the width and length of M₃ equal to that of M₄ and by using the same biasing voltage for M₃ as for M₄, i.e., by setting V_(M3)=V_(M4), g_(m3) becomes

$g_{m\; 3} = {g_{m\; 4} = {2\left( {1 - \frac{1}{\sqrt{X}}} \right)f_{clk}{C_{clk}.}}}$

As discussed previously, depending upon the dimensions of M₃, M₂, and M₁, the transconductance of the differential pair g_(m1,2) in becomes

g_(m1,2)=Ag_(m3).

It thus follows that the VCO start-up condition (g_(m1,2)·R≧1) using a plurality of VCO stages 700 becomes: 2A(1−l/sqrt(X))≧1. Such a VCO startup condition is independent of power supply variations and insensitive to device parameters and capacitance values. It only depends on the ratio of the values of device widths. Therefore, the resulting start-up is independent of process and temperature variations and of power supply noise.

FIG. 8 shows a VCO 800 that includes a plurality of VCO stages 700. As discussed with regard to FIG. 5 d, adjustable switched-capacitor circuits 405 as shown in FIG. 7 may be adjusted through application of a control voltage to varactors. Thus, each stage 700 receives a control voltage (V_(control)) to control the output frequency produced at a pair of differential output nodes 810. The biasing circuit 600 shown in FIG. 7 need not be repeated separately for each stage but instead a single biasing circuit may be common to all the stages.

The self-compensating oscillation and startup properties of VCO 800 have many applications. For example, VCO 800 may be incorporated into a phase-locked loop (PLL) 900 as shown in FIG. 9 that includes a phase detector 910, a loop filter 915, and a loop divider 920. VCO 800 responds to the control voltage V_(control) as discussed above to control its output frequency. Advantageously, this output signal frequency will not depend on the particular semiconductor process corner used to manufacture the integrated circuit fowling VCO 8900. Moreover, the startup conditions will not depend on operating temperature changes or process variations.

Consider the advantages of such frequency and startup independence as compared to conventional VCOs. In general, it is convention to generate the biasing tail current I in response to a bandgap voltage. Different corners, temperatures, and supply voltages require different transconductances and hence require different biasing currents to start the oscillation. For example, if the fast corner at room temperature requires a current of I_(ss) to start the oscillation, then a slow or hot process corner would, in general, require substantially more current for the start-up (as much 50% or 100% more current than I_(ss)). To ensure that oscillation starts in the worst case (usually a hot and slow corner), VCOs based on prior technologies use the worst-case (maximum) biasing current, even though other corners (for example, nominal and fast corners at room temperature) need substantially less currents for start-up, thus wasting considerable power. In contrast, the biasing currents of the VCOs and free running oscillators discussed herein are self-compensating. For example slow and hot corner uses more biasing current (more power) than the nominal corner. And the nominal corner, in turn, uses more biasing current than the fast corner, thus saving considerable power. Furthermore, in contrast to prior technologies, the start-up condition for the VCOs and ring oscillator discussed herein is insensitive to the power supply noise.

It will be appreciated that the techniques and concepts discussed herein are not limited to the specific disclosed embodiments. For example, the present invention encompasses other VCO topologies such as delay variation by positive feedback and delay variation by interpolation. In addition, these techniques and concepts may be implemented in a wide variety of technologies such CMOS, bipolar, SiGe, and GaAs. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. For example, the voltage-controlled oscillator disclosed herein may be used in other applications besides phase-locked loops. The appended claims encompass all such changes and modifications as fall within the true spirit and scope of this invention. 

I claim:
 1. A ring oscillator, comprising: a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source, the current source sourcing the tail current responsive to a bias voltage; and a bias circuit configured to generate the bias voltage such that a transconductance for each transistor in the differential pairs is proportional to a factor that is a function of a ratio of transistor widths within the bias circuit.
 2. The ring oscillator of claim 1, wherein the bias circuit includes a PMOS current mirror mirroring current into a first NMOS transistor and a second NMOS transistor.
 3. The ring oscillator or claim 2, wherein the PMOS current mirror comprises a first PMOS transistor and a second PMOS transistor.
 4. The ring oscillator of claim 3, wherein a source of the second NMOS transistor couples to ground through a resistor.
 5. The ring oscillator of claim 3, wherein a length L for each of the first and second PMOS transistors and for each of the first and second NMOS transistors is the same.
 6. The ring oscillator of claim 5, wherein a width W for each of the first and second PMOS transistors and for the first NMOS transistor is the same, and wherein a width for the second NMOS transistor has a width of X multiplied by W, where X is a proportionality factor greater than
 1. 7. A voltage-controlled oscillator (VCO), comprising: a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source, the current source sourcing the tail current responsive to a bias voltage, wherein each transistor in the differential pair couples to a power source through a corresponding switching-capacitor circuit; and a bias circuit configured to generate the bias voltage such that a transconductance for each transistor in the differential pairs is proportional to a factor that is a function of a ratio of transistor widths within the bias circuit.
 8. The VCO of claim 7, wherein the bias circuit includes a PMOS current mirror mirroring current into a first NMOS transistor and a second NMOS transistor.
 9. The VCO of claim 8, wherein the PMOS current mirror comprises a first PMOS transistor and a second PMOS transistor.
 10. The VCO of claim 9, wherein a source of the second NMOS transistor couples to ground through a resistance.
 11. The VCO of claim 10, wherein a length L for each of the first and second PMOS transistors and for each of the first and second NMOS transistors is the same.
 12. The VCO of claim 11, wherein a width W for each of the first and second PMOS transistors and for the first NMOS transistor is the same, and wherein a width for the second NMOS transistor has a width of X multiplied by W, where X is a proportionality factor greater than
 1. 13. The VCO of claim 12, wherein each corresponding switched-capacitor circuit includes a varactor having an adjustable capacitance responsive to a control voltage.
 14. The VCO of claim 10, wherein a switched-capacitor circuit provides the resistance.
 15. A phase-locked loop (PLL), comprising: a phase detector configured to compare the phase between a divided signal and in input signal to provided a phase detector output; a loop filter to filter the phase detector output to provide a tuning voltage; and A voltage-controlled oscillator (VCO) including a plurality of differential inverters coupled to form a loop, each differential inverter having a differential pair of transistors configured to steer a tail current from a current source, the current source sourcing the tail current responsive to a bias voltage, wherein each transistor in the differential pair couples to a power source through a corresponding switching-capacitor circuit having an adjustable capacitance responsive to the tuning voltage; and a bias circuit configured to generate the bias voltage such that a transconductance for each transistor in the differential pairs is proportional to a factor that is a function of a ratio of transistor widths within the bias circuit.
 16. The PLL of claim 15, wherein the bias circuit includes a PMOS current mirror mirroring current into a first NMOS transistor and a second NMOS transistor.
 17. The PLL of claim 16, wherein the PMOS current mirror comprises a first PMOS transistor and a second PMOS transistor.
 18. The PLL of claim 17, wherein a source of the second NMOS transistor couples to ground through a resistance generated by a switched-capacitor circuit.
 19. The VCO of claim 10, wherein a length L for each of the first and second PMOS transistors and for each of the first and second NMOS transistors is the same.
 20. The VCO of claim 11, wherein a width W for each of the first and second PMOS transistors and for the first NMOS transistor is the same, and wherein a width for the second NMOS transistor has a width of X multiplied by W, where X is a proportionality factor greater than
 1. 